Numerous cases of cytoplasmic inheritance depend upon particles of bacterial or viral origin. The inheritance of the killer trait in Paramecium depends upon two such particles. Kappa (of bacterial origin) contains an extrachromosomal element (similar to the bacterial plasmids) which can mature into virus-like structures consisting of about half DNA and half protein. The maturation process results also in the formation within kappa of free capsomere-like elements as well as a unique structure, the refractile or R body. The R body is a long, thin ribbon of proteins wound into a compact roll and capable of suddenly unrolling. The virus-like structures and the R body are implicated in the toxic action kappa on kappa-free sensitive paramecia. Furthermore the presence of kappa and its plasmids induces paramecia to become specifically resistant to the toxin. It is proposed that an effort be made to elucidate the relationships between paramecia, kappas, virus-like structures, R bodies and the capsomere-like elements. Of particular interest is the role of nuclear genes in maintaining kappa, the nature of killing and the mechanisms of sensitivity and resistance. We propose to start our investigation by obtaining a series of nuclear gene mutations which affect these processes. Experiments using more direct chemical, serological and electron microscopic techniques will follow.